The present invention relates to an imaging system which is to be panned across a viewed scene, and more particularly, but not exclusively to such a system for mounting onboard an aircraft. In the context of this application imaging system is used to define a system which generates data relating to a received image, but the term does not necessarily require that the data be subsequently presented as an image to an operator.
In certain applications it is particularly desirable to use a long linear array of detector elements and to scan an image orthogonally across that array of detector elements such that a complete image frame can be derived from the output of the array of detector elements. Such systems typically find application in high quality infra-red imaging systems. The linear array will often comprise more than a single row of elements and may typically comprise 10 rows of element, permitting time delay and integration (TDI) to be employed.
A common display format will comprise 768 vertical columns of 576 lines each. It has previously been proposed to have 480 element linear detectors, or pseudo linear detectors (see below), arranged vertically relative to an image, with the image being scanned horizontally across the 480 elements. The reasons for such proposals arises because employing 480 element linear detectors, which are commonly available, enables 480 out of a possible 576 lines of an image to be generated. The array is read out 768 times corresponding to the 768 columns of the display. In many applications a 480 row image is acceptable.
It has now been realised that it is advantageous in certain applications to employ long linear arrays of detector elements comprising approximately 768 elements for, although such long linear arrays are usually more expensive, when the image is scanned vertically over such a linear array arranged horizontally, then the array only need be read out 576 times to obtain a full image frame. The significance of this is that for a given standard frame rate the stare time available for each element is increased by approximately one third. Furthermore because the scan is vertical scan interlaces can be performed. This enables a considerable increase in the performance of an infra-red (IR) imaging systems where stare time limits performance. Similar performance improvements can be made with other display formats, for example, 640 vertical columns, of 480 lines.
An imaging system employing a horizontal long linear array of detector elements also has a lower inherent latency arising from the imaging system operating with the same raster scan format as a standard display, or video storage means. Image data from the first complete line of the display is available to the display, video storage means, or image processing means, whilst the remainder of the image is still being generated. This is to be contrasted with systems where the image is generated by scanning across a vertical array of detector elements where no row of the image is complete until the data from the last column of the image has been obtained. This is particularly important in certain applications, such as where the imaging system is mounted on an aircraft and can experience rapid changes in attitude.
A further advantage of using a horizontal array of detector elements is that it reduces the required scan angle. This is advantageous because the performance of such scanning mechanisms normally decreases as the scan angle increases.
Despite the advantages offered by using a horizontal long linear array of detector elements, and scanning an image orthogonally over that array, there is an inherent problem if it is desired to pan the imaging system in a direction having a component perpendicular to the scan direction, because panning causes the image to drift sideways across the detector during composition of an image frame.
In conventional systems, (where the direction of pan is the same as the scan direction), pan rate can be compensated for by either altering the read out time or the horizontal scan rate, if the linear array of detector elements employs a time delay and integration (TDI) technique, where charge generated by a space coordinate of the image is clocked along successive detector elements in a TDI channel of the array of detector elements in synchronisation with the image passing along that channel, such that charge generated by a point on the image is summed with charge generated by the same space coordinate being incident previously on earlier elements of the channel, then controlling the scan rate or clock rate in dependance on pan rate will ensure that synchronisation is maintained between a point on the image moving across the array and the corresponding packet of charge generated moving along the detector elements of a channel. This is not possible where the image moves sideways across the detectors comprised in a TDI channel.
The above poses a particular problem in such applications as search and track, where an infra-red imager is panned across the scene ahead of an aircraft normally to identify objects in an otherwise substantially homogenous environment such as the sea. In such an application TDI processing is particularly important to provide maximum sensitivity whilst maintaining resolution but it is also desired to maximise stare time by employing a long linear array of detectors and scanning vertically across the array to provide a normal image format.